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Arsenic speciation and localization in horticultural produce grown in a historically impacted mining region Andy Meharg, Gareth John Norton, Claire M. Deacon, Jörg Feldmann, paul jenkins, Christina Baskaran, and Adrien Mestrot Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es400720r • Publication Date (Web): 20 May 2013 Downloaded from http://pubs.acs.org on May 23, 2013

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Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Environmental Science & Technology

O HO

As OH OH

Arsenate

O HO

As OH OH Arsenite

HO

As OH

CH 3 Monomethylarsonic acid

ACS Paragon Plus Environment

O CH As OH 3 CH 3 Dimethylarsinic acid


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Arsenic speciation and localization in horticultural produce grown in a historically impacted mining region

Gareth Norton1, Claire Deacon1, Adrien Mestrot1,2, Joerg Feldmann2, Paul Jenkins3, Christina Baskaran3, Andrew A. Meharg4*

1. School of Biological and Environmental Sciences, University of Aberdeen, Cruickshank Building St Machar Drive, Aberdeen AB24 3UU, UK. 2. Department of Chemistry, School of Physical Sciences, University of Aberdeen, Meston Building, AB243UE, UK. 3. Food Standards Agency, Aviation House, Kingsway, London,WC2B 6NH, UK. 4. Institute for Global Food Security, Queen’s University Belfast, David Keir Building, Malone Road, Belfast, BT9 5BN, UK.

* Corresponding author: email – [email protected]; phone – +44 (0) 2890975413; fax – +44 (0) 2890976513.


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Abstract A field and market basket study (~1,300 samples) of locally grown fruits and vegetables from historically mined regions of South-West (SW) England (Cornwall and Devon), and as reference, a market basket study of similarly locally grown produce from the NorthEast (NE) of Scotland (Aberdeenshire) was conducted to determine the concentration of total and inorganic arsenic present in produce from these two geogenically different areas of the UK. On average 98.5% of the total arsenic found was present in the inorganic form. For both the market basket and field survey the highest total arsenic was present in open leaf structure produce (i.e. kale, chard, lettuce, greens and spinach) being most likely to soil/dust contamination of the open leaf structure. The concentration of total arsenic in potatoes, swedes and carrots was lower in peeled produce compared to unpeeled produce. For baked potatoes, the concentration of total arsenic in the skin was higher compared to the total arsenic concentration of the potato flesh, this difference in localization being confirmed by Laser Ablation – ICP-MS. For all above ground produce e.g. apples, peeling did not have a significant effect on the concentration of total arsenic present.


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Introduction Arsenic can become elevated in agronomic crops due to anthropogenic pollution; primarily contaminated groundwater irrigation (1) and, more widely, base and precious metal mining environmental contamination (2-7). Grain crops have been investigated in most detail, with respect to arsenic accumulation, as rice is particularly efficient at taking up arsenic and translocating it to the grain (8), while horticultural crop produce remains relatively understudied with few systematic surveys (9-12). This is perhaps understandable as cereal crops tend to be dietary staples, whereas, with the exception of potatoes and equivalent starch staples (cassava, yams, sweet potato etc.), horticultural products are not normally dietary staples, and are more variable in nature and cultivation than grains.

Terrestrial agronomic produce tends to be dominated by inorganic arsenic, with dimethylarsinic acid (DMA) being the other arsenic species routinely found (1). Inorganic arsenic is a class-one, non-threshold carcinogen whose chronic exposure leads to bladder, lung and skin cancers (13). It has been demonstrated that arsenic is not methylated within plants (14), therefore the accumulation of organic arsenic is dependent on the environment the plants are grown in. Previous legislative and regulatory focus on inorganic arsenic has been on drinking water as a human exposure source, with water standards in the European Union being in place (15). There are no food standards for either total or inorganic arsenic for foods for the EU (16). However, it is increasingly recognised that some food sources can be a significant route to inorganic arsenic exposure, as described in the report on this subject area by the European Food Safety


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Authority (EFSA) (16). The consequences for chronic human exposure to DMA and other organic arsenic species is currently uncertain, but organic arsenic compounds exposure to humans is thought to be of less concern as compared to inorganic arsenic (16). As mining impacted regions elevated in arsenic represent the worst-case scenarios for contamination of terrestrial food-chains throughout the world (2, 17, 18), it is important to assess total arsenic and it’s speciation and localization in crops to not only ascertain what particular produce maybe problematic, but to also determine if food preparation (i.e. peeling) can lead to amelioration of contamination.

Here we investigate the nature and extent of arsenic contamination in horticultural produce in the historic mining region of the SW of England where there are concerns regarding the lasting impacts of arsenic contamination from historical base (arsenic, copper, lead, zinc) and precious (silver) metal mining in that region (17). Some of the densest areas of ancient mining are now centres of horticultural produce, highly significant in local food consumption as well as producing crops (brassicas and early potatoes) that are widely distributed nationally. As a comparison a second survey was performed in the NE of Scotland, an area that has generally lower soil arsenic concentrations. Total arsenic, arsenic speciation and laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) analyses were performed on both market basket surveyed fruit and vegetables, as well as field surveyed produce where arsenic contamination could be related back to the soil it was grown in. Furthermore, the effect of food preparation, peeling and baking on arsenic levels was also studied.


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Materials and Methods Sample collection The survey sampled locally produced fruit and vegetables in retail outlets, from Cornwall and Devon (both SW Britian), and Aberdeenshire (NW Britian), as well as field crops and soil from the SW. The aim of the basket survey was to select locally produced from a wide variety of retailers. For the basket survey local farm shops, greengrocers, “pick your own”, supermarkets, honesty boxes and farmers’ markets were targeted; only samples of local fruit and vegetable produce were purchased (confirmed by either labelling of produce or asking the supplier). Either 100g or 5 individual vegetables or fruits were sampled (which ever was greater). All samples were washed in a kitchen sink using local tap water, to a thoroughness normally used in food preparation. Once cleaned the samples were then finely diced in a food processor (which was cleaned between processing each sample). The samples were then frozen before further processing. For items eaten with and without skin (potatoes, root vegetables, apples, pears etc.), duplicate samples were collected and one was peeled and the other unpeeled and peeled, to reflect different dietary exposures.

The data, with exception of peeled versus unpeeled comparisons, presented here are based on the preparation most commonly consumed, i.e. unpeeled for apples and courgettes, while peeled are potatoes, swedes, parsnip, carrots, beetroots and squashes, with the other preparation method (i.e. peeled apples, unpeeled potatoes etc.) referred to as the “alternative preparation”. The SW basket survey consisted of 630 samples with


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207 alternative preparations. The NE basket survey consisted of 190 samples with 69 alternative peeling preparations.

To establish the link between soil arsenic and the concentration of total arsenic in vegetable and fruit produce, farmers’ fields in the SW geographic areas were sampled, with the farmer’s permission, for fruit and vegetables in season. The fields were selected on a basis of having harvestable produce at the time of sampling, not systematic sampled based on underlying geological, but do reflect direct entry of produce into the human food-chain. A transect 20 m long, sampled at 5 m intervals, was taken through the centre of the field and soil (top 20 cm, sampled using a stainless steel corer) and a sample of the fruit or vegetable was taken at each location. These 5 soil and 5 produce samples were bulked to give one soil and one produce sample per field. Produce samples were prepared as for the basket survey. Soil samples were oven dried and 2 mm sieved before analysis. For the SW campaigns, a total of 174 soil samples were analysed along with corresponding crops, as well as 56 alternative preparations for produce eaten either peeled or unpeeled.

Twenty potato samples were selected for the determination of total arsenic in the skin and flesh of baked potatoes. Where possible the potatoes were classified as floury (n=8) or waxy (n=6) based on the information from the seller or farmer. For each sample three potatoes were washed as described earlier, and then baked at 180oC for 90 minutes. After the potatoes had cooled down the flesh was scooped out of the skins. To compare the


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concentrations of arsenic in the flesh and the skin a paired t-test was performed on the matching skin and flesh.

Total arsenic analysis Pureed produce samples were accurately weighed into 50mL polyethylene centrifuge tubes and then oven dried at 70oC and the moisture content determined. The weight of wet material was to give an estimated dry weight (accurately weighed) of between 0.20.3g. For digestion 2.5 mL of concentrated nitric acid was added to each sample and then incubated overnight. Trace reagent analysis grade reagents were used throughout. Prior to microwave digestion 2.5 mL of hydrogen peroxide was added to each tube and then the samples were digested using a Microwave Accelerated Reaction System (MARS ,CEM). The digestion parameters were a 5 min. ramp to 55oC and held for 10 min., a 5 min. ramp to 75oC and held for 10 min., a 5 minute ramp to 95oC and held 30 min. Samples were then made up to 50 mL and accurately weighed. Each analytical batch was accompanied by a minimum of 1 reagent blank, 1 acid blank spike, and 4 certified reference materials (CRM): 1x IC-INCT-MPH-2 mixed Polish herbs, 1x NIST-1568a rice flour, 1x CTAOTL-1 Oriental tobacco leaves and 1x NCS ZC73012 cabbage.

Sieved soil samples were weighed (0.1g) into glass digest tubes and 2.5 mL of concentrated nitric acid was added to each tube. The samples were left overnight with the acid. Hydrogen peroxide (2.5 mL) was added to each tube, the digest tubes were then transferred to a digest-heating block set at 100oC, after 1 h the temperature was increased to 120oC, then after 1 h the temperature was increased to 140oC. The samples were then


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digested for 4 h. The samples were transferred to 15 mL centrifuge tubes and made up to 10 mL, followed by a 1:10 dilution of the digest. For each soil batch; 1 reaction blank, 1 spike and 1 soil CRM (NCS DC73319) were used.

Total arsenic analysis was performed by ICP-MS (Agilent Technologies 7500). Rhodium (10 µg L-1) was run on an external line as the internal standard. The arsenic standards ranged from 0.1 – 300 ug L-1. The elements measured were arsenic (m/z 75) and rhodium (m/z 103 – internal standard). Possible argon chloride interference (none was found) on m/z 75 was monitored directly using selenium isotopes m/z 77 and m/z 82.

Aluminum analysis To determine if the concentrations of arsenic in the samples where related to the possible soil contamination aluminum concentration in the samples was determined. The concentration of aluminum is high in soils and generally low in plant material. The concentration of aluminum (19) in 82 “open leaf structure” vegetables was determined. Open structure vegetables are a group of vegetables based on their overall morphology, they have a large surface area in relation to mass. They include vegetables such as spinach, kale, greens, lettuce and chard. Approximately 0.2 g of sample was weighed accurately into glass digestion tubes. The digest reagent was prepared by dissolving 4.6 g lithium sulphate in 300 mL of concentrated sulphuric acid. A total of 2.4 mL of this reagent was added to the digest tube along with 2.1 mL of 100 volumes hydrogen peroxide. Trace reagent analysis grade reagents were used throughout. A marble was placed on top of the digest tube to assist reflux and the block digester was heated up to


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365oC and held there for 2 h. Once cooled the digest was poured into 50 mL centrifuge tubes and the glass digestion tubes rinsed with deionised water 3 times into the centrifuge tubes then made up to 50 mL by volume with deionised water. Three replicates each of blanks and certified reference materials IC-INCT-MPH-2 mixed Polish herbs, CTAOTL-1 Oriental tobacco leaves and NCS ZC73012 cabbage were used as quality controls and all the samples, blanks and CRM’s were randomised prior to analysis. For aluminium analysis samples, blanks and CRM’s were diluted 1:1 with 10000 mg L-1 lanthanum chloride. Standards in the range 0.2-5 mg L-1 were made from 1000 mg L-1 Aluminium stock solution in a matrix of 1:1 4.8%sulphuric acid: 10000 mg L-1 lanthanum chloride to be matrix matched with the final diluted sample. Analysis was performed on a Perkin Elmer AAnalyst100 Atomic Absorption Spectrophotometer (AAS) with a nitrous oxide/acetylene flame using a 5 cm burner head.

Laser Ablation – Inductively Coupled Plasma – Mass Spectroscopy Apples, beetroots, carrots, parsnips and potatoes were selected from SW and NE samples. After sampling, all produce was stored fresh at 5oC for up to 1 week before analysis. Whole produce samples were thoroughly washed with tap water, as per typical food preparation. Samples were then diced (~1 cm3) and 3 skin containing dice samples were randomly chosen per item. The cubes were then flash frozen using liquid nitrogen and stored frozen at -20oC until sectioning. Sections were prepared from each batch at -15oC using a cryostat (Model OTF including microtome 5030, Bright). A random cube from each batch was mounted on a sample holder and fixed using Tissue-Tek O.C.T. paste (Sakura Finetek Europe). A microtome was used to prepare thin-sections of up to 35µm


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depth. The slices were thaw-mounted onto microscope slides and allowed to air-dry at room temperature (for 1-2 hrs depending on the thickness of the slice), and subsequently, stored frozen at -20oC until analysis. Each sample analysed by LA-ICP-MS (New Wave model UP-213), interfaced with ICP-MS (Agilent Technologies, model 7500c) and masses (m/z) carbon [13], sulphur [34], silicon [29], arsenic [75], and selenium [77, 82] monitored. Silicon was monitored in order to delineate the slide from the sample. Carbon and sulphur were used to verify that the thickness of the ablation was relatively constant. Three ablation lines were performed for each section and the data averaged for plotting. The ablation always started at least 200µm outside the sample, on the skin side. As the structure of each sample will determine (density and porosity) how much of it is ablated, LA-ICP-MS cannot be used for quantitative analysis.

Arsenic speciation Samples were oven dried at 70oC and the moisture content of the produce determined. Dried samples were then powered by milling and approximately 0.2g of dried produce was accurately weighed out into 50 ml polyethylene centrifuge tubes, 10 mL of 1% nitric acid was added to each sample and then left overnight. Trace reagent analysis grade reagents were used throughout. Microwave extraction was performed as for total arsenic analysis. Each analytical batch was accompanied by a minimum of 1 reagent blank, and 2 certified reference materials (rice flour NIST-1568a and cabbage NCS ZC73012). After extraction samples were centrifuged at 10,000g for 10 min. at 4oC and 900 µL of supernatant was removed. To the supernatant 100 µL of hydrogen peroxide was added and the samples then were stored at 4oC overnight. The addition of hydrogen peroxide


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oxidises the arsenite to arsenate, this is done to as the DMA and arsenite peaks can overlap. This means that only total inorganic can be determined not the individual inorganic arsenic species. Arsenic speciation was determined using HPLC-ICP-MS (Agilent Technologies 7500). The mobile phase was an ammonia phosphate buffer (6.6 mM ammonium dihydro-phosphate, 6.6 mM ammonium nitrate, pH 6.2), 100 µL of the sample was injected into a PRP -X100 anion exchange column. Rhodium (10 µg L-1) was introduced on an external line as the internal standard, m/z 75, arsenic and m/z 103, rhodium were monitored. DMA standards (0.5 µgL-1 – 75 µgL-1) were used to quantify peaks and a 10 µg L-1 standard mix of arsenate, arsenite, DMA and MMA was used to determine retention time. Arsenic species are reported in dry weights to make them directly comparable with literature studies. Every individual sample analysed is reported and the percentage inorganic arsenic found in classes of vegetables and fruits can then be applied directly to total arsenic in produce reported as fresh weight as dietary consumption data is reported in fresh weight.


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Results Total arsenic concentrations – basket survey The average recovery for the cabbage (97.3%) and the tobacco (97.8%) CRMs were very good, and the recoveries for the herb and rice CRMs were over 80% (SI Table 1). This variation in CRM is to be expected as the efficacy of acid digestion is dependent on arsenic speciation in the CRM, and this will vary between CRMs. The soil CRM (NCS ZC73007) total arsenic was 14.17 mg kg-1 (s.e.m 0.12 mg kg-1; n = 12), a recovery of 78.7% (certified value; 18 ± 2 mg kg-1).

The total arsenic value for every sample (including alternative preparations) is presented in SI Table 2 for the SW basket survey. For the individual samples (prepared in the way that the produce is most commonly consumed) from the SW basket survey 22.5% of the arsenic concentrations were below the LoD (2.0 ng g-1). For the produce for which at least three independent samples were collected the mean, median, minimum and maximum total arsenic value for that produce class are presented in SI Table 3. The distribution of the total arsenic concentrations for the classes is presented in Figure 1. For most classes of produce (27 out of 33) from the basket survey the mean total arsenic concentration did not exceed 25 ng g-1 arsenic (fresh weight), with only kale, chard, lettuce, greens, spinach and mixed leaf salad having a mean total arsenic concentration above 25 ng g-1arsenic (fresh weight). The only produce which exceeds a median of 25 ng g-1arsenic (fresh weight) was mixed leaf salad. A total of 17 of the produce classes had a maximum total arsenic value that was above 25 ng g-1 (fresh weight).


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For the NE basket survey the total arsenic value for every sample (including alternative preparations) is presented in SI Table 4. For the individual samples (prepared in the way that the produce is most commonly consumed) from the NE basket survey 51.9% of the arsenic concentrations were below the LoD (2.0 ng g-1). For the produce for which at least three independent samples were collected the mean, median, minimum and maximum total arsenic value for that produce class are presented in SI Table 5. The total arsenic concentration distribution for the classes in presented in SI Figure 1. For all the classes of produce from the basket survey the mean (and median) total arsenic concentration did not exceed 25 ng g-1 arsenic (fresh weight). Only 1 class of produce (kale) had a maximum total arsenic concentration greater than 25 ng g-1 (fresh weight).

For nine different classes of fruit and vegetables a comparison was conducted between fruit and vegetable with unpeeled and peeled (Figure 2). Statistical analysis was performed using a paired t-test for each produce category. For apples (n=17), beetroots (n=14), courgettes (n=22), cucumbers (n=3), parsnips (n=9) and squashes (n=7) there was no significant difference between the preparation styles. For carrots (n=32), potatoes (n=79) and swedes (n=23) there was significantly (at the 5% level) more arsenic in the vegetables that had not been peeled.

Total arsenic concentrations – field survey For the SW field survey soil arsenic concentrations ranged from 11.4 - 438.6 mg kg-1, with an average concentration of 110.3 mg kg-1. All the soil arsenic values and associated


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produce total arsenic concentrations are presented in SI Table 6. Good correlations between produce total arsenic (as normally eaten) and soil arsenic were identified for a number of produce (Figure 3 A-H).

There is very strong evidence of a positive association between arsenic in potatoes (both peeled and unpeeled) and their surrounding soil (Table 1). The association was also significant at the 5% level in cauliflower and broccoli (combined), cabbages and peeled root vegetables (Table 1). In unpeeled root vegetables, it is just below this level of significance (Table 1). The associations in open-leaf vegetables and soft fruit were also below the 5% significance level (Table 1).

Aluminum/arsenic comparisons in open leaf structure produce For aluminum CRMs the average recovery was very variable for the different CRMs. For the cabbage CRM the recovery was 114.4% ± 6.4%, for the herb CRM the recovery was 61.8% ± 8.6% and for the tobacco CRM the recovery was 28.5% ± 8.9%. This indicates that the CRM speciation of aluminium differed greatly, and that the results must be considered with this in mind. The LoD for aluminium was 182.3 mg kg-1. All samples were run as a single batch. Out of the 82 samples analysed 36 samples were below the LoD. For statistical analysis these 36 samples were allocated a half LoD concentration in subsequent interpretation. The concentration of aluminium in the samples analyzed is presented in SI Table 7. The correlation indicates a significant association, at the 0.1% level, (SI Figure 2) between the concentration of total arsenic in the samples and the concentration of aluminium (P

Arsenic Speciation and Localization in Horticultural ... - ACS Publications

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